In maintaining the structural integrity of a pipeline, it is important to be able to detect and repair anomalies in the pipeline wall. For present purposes, anomalies include defects such as cracks, pitting, corrosion and dents. In particular, it is important to be able to determine the location of the anomaly, the type of the anomaly and the geography of the anomaly (i.e. its shape and size).
When it is desired to test the structural integrity of a pipeline, a pig may be placed in the pipeline where it is then propelled through the pipeline by the product in the line, such as oil or gas. The pig is typically fitted with urethane "cups" near its front end to seal the annulus between the pig and the pipeline wall, thus preventing pipeline fluid from flowing through the annulus. Pressure behind the pig builds up to the point that the pig begins to move. The differential pressure created across the front and rear of the cups keeps the pig moving through the line.
As the pig moves through the pipeline, it can gather data concerning anomalies in the pipeline wall. If this information can be measured by the pig, stored and later retrieved, it can then be analyzed so as to permit remedial action to be taken.
It is known to use a pig in combination with magnetic flux leakage (MFL) technology to detect defects in pipeline walls. The principle of MFL is based on magnetizing the pipe wall and using sensors to measure the leakage field generated by anomalies in the pipe wall material. For example, if no anomalies are present, the magnetic field will be continuous and there will be no leakage to be detected. If there is an anomaly such as a crack, the magnetic field in the pipeline wall will be interrupted and the stray fields which are generated will provide useful information concerning the anomaly.
Although there are pigs, including pigs manufactured by the assignee of the present application, which do an adequate job of detecting and measuring anomalies in pipeline walls, such pigs have limitations which can impact upon the quality of the measurements obtained. The main drawback with the prior art pigs is that they are not capable of generating sufficient magnetic power for transmission through relatively thick pipeline walls. In any situation, the requisite magnetic field is proportional to the thickness of the pipe wall; that is, the thicker the wall is, the stronger the field has to be to penetrate the wall. The prior art pigs are, for the most part, limited to pipeline walls of approximately 12 mm (about 1/2 inch) thickness given that these pigs are not designed to generate the requisite magnetic field for thicker pipeline walls.
Another limitation of the prior art pigs is the spacing (and therefore the number) of sensing elements which are mounted around the circumference of the pig. For example, if the distance between the sensing elements is 15 mm, as is the case with the previous generation of pigs manufactured by the assignee of the present application, then a defect could be up to 12 mm in diameter and not always be detected correctly.
The positioning of the sensors in relation to the pipeline wall also limits the depth of defect that can be sensed. The prior art methods of mounting the sensing element, the element which actually measures the magnetic field, place limitations on how close the sensing element can be placed in relation to the pipeline wall. This results in a less than optimum signal-to-noise ratio and, hence, a less than optimum MFL reading. For example, a defect with less than 10% wall thickness material loss typically cannot be picked up by these sensors.
Yet another limitation of the prior art is the length of pipeline that the pigs can inspect. This limitation is dictated not only by the amount of data that the recording units can collect and the amount of battery power available to keep the recorders going, but also by the fact that the mechanical components wear while riding for a prolonged period of time against the pipe wall. The components which transmit the magnetic field to the pipeline wall and the sensors themselves are in constant contact with the pipeline wall and therefore tend to wear over time.
There has therefore developed a need for a pig which is capable of detecting and diagnosing anomalies in relatively thick pipeline walls and which is capable of achieving a higher than previously attained level of accuracy in the measurement of magnetic flux leakage in any size of pipeline wall. There has also developed a need for a pig on which the components which generate the magnetic field and the components which measure the MFL are less prone to wear than the pigs of the prior art so that the pigs may be more suitable for long run applications or a series of short runs without the need for refurbishments between runs.